This is a revised competing renewal application for K24 funding. The candidate is a pediatric hematologist and geneticist, with basic science training in biochemistry and molecular biology, and a clinical research background in pediatric blood, lipid and vascular disorders. The primary goal for this award is to provide protected time for mentoring in the setting of several ongoing patient-oriented research projects in pediatric blood disorders. In the long run, the goal is to establish the PI as a senior mentor for students, residents, fellows, and junior members interested in patient-oriented research in pediatric hematology and genetics. The study environment is the Pediatric Hematology/Oncology program of Children's Hospital Boston and Dana Farber Cancer Institute. Opportunities for mentoring include Harvard undergraduates, local and visiting medical students, residents and fellows not limited to those in hematology, and junior faculty members launching careers in academic medicine and patient oriented research. In this revised application, the focus is on "bedside to bench" analysis of the genetic pathophysiology of blood disorders, divided into two projects. The first aim/project is to further characterize the rare human disorder, thiamine-responsive megaloblastic anemia, due to defects in the gene for high-affinity thiamine transporter, SLC19A2. In the first K24 support period, the candidate's lab identified the causative gene and many patients' mutations. Further studies of mutations of this SLC19A2 gene will be performed on recently identified patients with TRMA, and related disorders. [3H]Thiamine uptake studies and stable isotope metabolic profiling with [1,2-13C]glucose, both methods developed in the initial grant period, will be pursued for cells from patients with interesting TRMA variants, including one with no detectable SLC19A2 mutation. A murine model of TRMA, developed in the first K24 period for pathophysiology studies not possible in humans, will be utilized for marrow studies that will further elucidate the pathophysiology of the disorder. Marrow transplantation from TRMA mice into normal will determine if the marrow defect is cell-intrinsic. In the second aim, candidate gene sequencing will be used to identify the cause of rare inherited blood diseases identified in the hematology program at Children's Hospital. Using this approach, several unique disorders of blood clotting and anemia have been studied.